The hydrocarbon dew point of a vaporised hydrocarbon-comprising stream, i.e. the temperature at which the first hydrocarbons in the stream condense, is a key parameter in many industrial applications. Over time, such condensed hydrocarbons can cause equipment fouling, which in turn may lead to downtime for cleaning or even replacement of equipment.
In general, the hydrocarbon dew point dictates the minimum temperature required in a process to avoid undesired condensation. One example of such processes is the transmission of natural gas through pipelines, where the hydrocarbon dew point is sub-zero and the aim is to prevent condensation of C2 to C5 hydrocarbons. Another example is the gasification of biomass, where a tar by-product is formed having a dew point in the order of 170° C. In for instance steam cracking of naphtha, hydrowax, gasoline or gasoil type feedstocks, the feedstock is exposed to very high temperatures in specific parts of the convection section of the furnace. Any non-vaporised liquids contacting the walls of the pipes in this part of the convection section would result in an almost instantaneous decomposition and coke formation. Therefore, it is required to maintain the temperature of the mixture of steam and vaporised hydrocarbon feedstock in these sections above the dew point of the hydrocarbons in the feedstock. Due to the nature of the hydrocarbons in the steam cracker feedstock, the dew point may be above 170° C.
Commercially available apparatuses for measuring hydrocarbon dew point temperatures are based on detection of the dew by light reflection from a mirror. A hydrocarbon vapour at a set temperature is sent over a highly polished steel surface/mirror, the central point of which is maintained below the temperature of the vapour. The mirror is conical in shape, with the central point being physically and thermally the lowest point. Light is focused on this central point and as soon as hydrocarbons condense on the mirror, the reflection pattern of the light changes. The temperature of the central point of the mirror is reported as the dew point temperature.
These commercially available apparatuses can measure dew points up to a maximum of 170° C. owing to temperature limitations of the electronics and fiber optic cables.
For steam cracking feedstocks dew points are typically much greater than 170° C., ranging up to 500° C. Modifying the commercial apparatuses to meet this temperature requirement would likely be difficult; therefore there is a need in the art for a new method for determining the dew point of a vaporised hydrocarbon feedstock.